Alert Updates and Cancellations

When an alert changes, issue a new alert that refers to the previous alert, instead of changing or removing the existing alert from your feed. After an appropriate amount of time (e.g. 24-48 hours), remove cancelled, updated, or expired alerts from your feed.

<msgType>Update or Cancel must include at least one <references> element.
As specified in the CAP standard, any alert message that updates a previous alert should use <msgType>Update</msgType> and set <references>code</references> to all previous related messages that haven't reached their <expires> date. The update or cancellation must apply to a non-expired alert.

There are three ways to cancel events, in order of preference:

Set an <expires> datetime for each event, with the message
description setting the expectation that this alert will end on its
own.

Issue a new <alert> with <msgType>Update</msgType>,
<responseType>"All Clear"</responseType>, and <expires> a short time in
the future.